Current collector, electrode structure, nonaqueous electrolyte battery, electrical storage device, and nitrocellulose resin material

ABSTRACT

A current collector which is suitable for discharging and charging at a large current density is provided. The present invention provides a current collector including a conductive substrate and a conductive resin layer provided on one side or both sides of the conductive substrate. The conductive resin layer contains a soluble nitrocellulose-based resin and a conductive material.

TECHNICAL FIELD

The present invention relates to a current collector suitable for chargeand discharge at a large current density, an electrode structure usingthe current collector, a non-aqueous electrolyte battery using theelectrode structure, an electrical storage device (example: electricaldouble layer capacitor, lithium ion capacitor), and to a solublenitrocellulose-based resin material for the current collector.

BACKGROUND ART

Conventionally, a non-aqueous electrolyte battery as represented by alithium ion battery have been receiving a demand for reduction incharging time. In order to meet such demand, the battery must be capableof being charged at high speed with a large current density. Inaddition, a non-aqueous electrolyte battery for automobiles have beenreceiving a demand for the ability to discharge high power at a largecurrent density, in order to provide the automobile with sufficientaccelerating property. When conducting charge and discharge at a largecurrent density, internal resistance of the battery is important toimprove the characteristics of maintaining battery capacity (high ratecharacteristics). The same can be said for other non-aqueous electrolytebatteries such as an electrical double layer capacitor and a lithium ioncapacitor, and an electrical storage device.

In general, the cause of the internal resistance are electricalresistance of the constituting material, interface resistance betweenthe constituting components, conductivity resistance of the ions whichare the charged particles in the electrolyte solution, electrodereaction resistance, and the like. Therefore, each of the resistanceneed be decreased in order to decrease the internal resistance. Amongthem, one of the most important internal resistance is the interfaceresistance between the conductive substrate comprising a metal foil (forexample, aluminum foil, copper foil and the like) and an active materiallayer. It has been known that one measure to decrease the interfaceresistance is to improve the adhesion in the interface.

In order to improve the adhesion between the current collector and theactive material layer, a technique in which the current collector iscoated with a conductive resin, and then a paste for forming an activematerial layer is coated thereon, has been conventionally proposed.Patent Literature 1 discloses a technique to form a conductive coatingby coating a positive electrode current collector with a conductivecoating including a conductive filler, vinyl butyral as a binding agent,and dibutyl phthalate as a plastcizing agent. Patent Literature 2discloses a technique to form a conductive coating including apolyacrylic acid or a copolymer of an acrylic acid and an acrylic acidester as a main binding agent, and a carbon powder as a conductivefiller.

CITATION LIST Patent Literature

[Patent Literature 1] JPH2-109256A

[Patent Literature 2] JPS62-160656A

SUMMARY OF INVENTION Technical Problem

However, there were cases where sufficient high rate characteristicscannot always be obtained, and the lifetime of the battery wasunsatisfactory. In order to decrease the interface resistivity betweenthe current collector (comprising a conductive substrate and aconductive resin) and an active material layer, not only the adhesionbetween the conductive resin layer of the current collector and theactive material layer being high is important, but the volumeresistivity of the conductive resin layer itself being low is alsoimportant. Here, adhesion directly affects the interface resistancebetween the conductive resin layer and the active material layer andaffects the lifetime of the battery, and the term “adhesion” means thatthere is no detachment even when the interface is permeated withelectrolyte solution and the layers are adhered firmly. In addition,concerning a positive electrode and a negative electrode of anon-aqueous electrolyte battery or a lithium ion capacitor, since thevolume of the active material in the active material layer changes bycharging and discharging, the active material easily becomes detachedfrom the active material layer. Therefore, detachment between the activematerial layer and the current collector easily occur. Since the volumechange in the active material at high rate charging and discharging islarge, high adhesion between the conductive resin layer and the activematerial layer is especially required. However, in conventionaltechniques, the adhesion between the conductive resin layer and theactive material layer was low, and the lifetime of the battery wasunsatisfactory. In addition, decrease in the volume resistivity of theconductive resin layer itself was not sufficient.

An object of the present invention is to provide a current collectorwhich can decrease the internal resistivity of a non-aqueous electrolytebattery, and can suitably be used for an electrical storage device of anon-aqueous electrolyte battery such as lithium ion secondary batteryand the like, electrical double layer capacitor, lithium ion capacitorand the like. The current collector can further improve the high ratecharacteristics and can elongate the lifetime of the battery. Thecurrent collector of the present invention can provide an electrodestructure having superior adhesion in the active material layer and theelectrode material layer, by further forming an active material layer oran electrode material layer. In addition, the non-aqueous electrolytebattery using the electrode structure, the electrode structure having anactive material layer formed on the current collector of the presentinvention, can achieve high rate characteristics by decreasing theinternal resistance of the current collector having the afore-mentionedcharacteristics. Further, the present invention provides an electricalstorage device such as an electrical double layer capacitor, lithium ioncapacitor and the like, which requires high-speed charging anddischarging of a large current. Such electrical storage device is usedin copy machines and automobiles.

Solution to Problem

The inventors of the present invention have investigated theconstitution of the current collector used for the positive electrode ofa lithium ion secondary battery. Accordingly, the inventors have foundthat high rate characteristics and lifetime of the battery can beimproved by using a resin containing a soluble nitrocellulose-basedresin and a conductive material as a resin layer, such resin beingapplied as a base coat when forming the active material layer, therebyleading to completion of the present invention. By using such currentcollector, improvement in high rate characteristics can be achieved anda non-aqueous electrolyte battery, an electrical double layer capacitor,or a lithium ion capacitor with elongated battery lifetime can beobtained.

That is, the present invention provides a conductive substrate and acurrent collector structured by applying a conductive resin layer on oneside or both sides of the conductive substrate, wherein the conductiveresin layer includes a soluble nitrocellulose-based resin and aconductive material.

Various embodiments will be provided hereinafter. The embodimentsexemplified hereinafter can be combined with each other.

Preferably, the soluble nitrocellulose-based resin of the currentcollector includes a soluble nitrocellulose and at least one resinselected from the group consisting of a melamine-based resin, anacryl-based resin, a polyacetal-based resin, and an epoxy-based resin.

Preferably, the soluble nitrocellulose-based resin of the currentcollector includes a melamine resin, a soluble nitrocellulose, and atleast one resin selected from the group consisting of an acryl-basedresin and a polyacetal-based resin.

Preferably, in the afore-mentioned current collector, the melamine-basedresin is contained by 5 to 55 mass %, and the soluble nitrocellulose iscontained by 40 to 90 mass %, when the total of the acryl-based resin,the polyacetal-based resin and the soluble nitrocellulose is 100 mass %.

Preferably, the surface of the conductive resin layer of theafore-mentioned current collector has a water contact angle of 80degrees or more and 125 degrees or less, preferably 90 degrees or moreand 110 degrees or less, when measured by θ/2 method in a thermostaticchamber at 23° C.

Preferably, an electrode structure comprises the afore-mentioned currentcollector and an active material layer or an electrode material layer onthe conductive resin layer of the current collector.

Preferably, in the electrode structure, the conductive resin layer ofthe current collector contains an active material.

Preferably, a non-aqueous electrolyte battery or an electrical storagedevice comprises the afore-mentioned electrode structure.

Preferably, a conductive resin material for a current collectorcomprises a soluble nitrocellulose-based resin, and a conductivematerial.

Preferably, in the soluble nitrocellulose-based resin material, thesoluble nitrocellulose-based resin contains a soluble nitrocellulose andat least one resin selected from the group consisting of amelamine-based resin, an acryl-based resin, a polyacetal-based resin,and an epoxy-based resin.

Preferably, in the soluble nitrocellulose-based resin material, thesoluble nitrocellulose-based resin contains a melamine-based resin, asoluble nitrocellulose, and at least one resin selected from the groupconsisting of an acryl-based resin and a polyacetal-based resin.

Preferably, in the soluble nitrocellulose-based resin material, themelamine-based resin is contained by 10 to 40 mass %, and the solublenitrocellulose is contained by 50 to 70 mass %, when the total of theacryl-based resin, the polyacetal-based resin and the solublenitrocellulose is 100 mass %.

Advantageous Effects of Invention

The current collector of the present invention is superior in the highrate characteristics, and is suitable for charging and discharging athigh speed with a large current density, and can achieve long lifetime.In addition, when the electrode structure, non-aqueous electrolytesolution battery such as lithium ion battery and the like, and anelectrical storage device such as an electrical double layer capacitor,lithium ion capacitor and the like are provided with the currentcollector of the present invention, they are superior in high ratecharacteristics, superior in charging and discharging at high speed witha large current density, and can achieve long lifetime.

DESCRIPTION OF EMBODIMENTS

The current collector of the present invention comprises a conductivesubstrate and a conductive resin layer provided on one side or bothsides of the conductive substrate, wherein the conductive resin layerincludes a soluble nitrocellulose-based resin and a conductive material.

The details are explained hereinafter.

<1. Conductive Substrate>

As the conductive substrate of the present invention, various metalfoils for a non-aqueous electrolyte battery, for an electrical doublelayer capacitor, or for a lithium ion capacitor, can be used. Inparticular, aluminum, aluminum alloy, copper, stainless steel, nickeland the like can be used. Among them, aluminum, aluminum alloy andcopper are preferable in view of the balance between the highconductivity and the cost. When aluminum foil is used as a positiveelectrode, it is preferable to use a pure-aluminum foil such as thosesatisfying JIS A1085 having high conductivity, since the presentinvention is aimed at improving the high rate characteristics. There isno limitation for the thickness of the conductive substrate, however,preferable thickness is 0.5 μm or more and 50 μm or less, morepreferably 7 to 100 μm, and further preferably 10 to 50 μm. When thethickness is less than 0.5 μm, the strength of the foil is insufficientand thus formation of the resin layer and the like becomes difficult. Onthe other hand, when the thickness exceeds 50 μm, other constitutingcomponents, especially the active material layer or the electrodematerial layer must be made thin for such excess in the thickness. Thiswould result in cases where sufficient capacity for the electricalstorage devices such as the non-aqueous electrolyte battery, theelectrical double layer capacitor, or the lithium ion capacitor cannotbe obtained.

<2. Conductive Resin Layer>

The conductive resin layer of the present invention (hereinafterreferred to as “resin layer”) is provided on one side or both sides ofthe afore-mentioned conductive substrate, and contains a solublenitrocellulose-based resin and a conductive material.

<2-1. Conductive Resin>

In the present invention, the conductive resin is a resin which containsa soluble nitrocellulose as a resin component. The conductive resin maycontain only the soluble nitrocellulose, or may contain the solublenitrocellulose and another resin. The soluble nitrocellulose is acellulose having a nitro group. When compared with other solublecelluloses such as carboxymethyl cellulose (CMC), application inelectrodes have been exemplified, however, there have been no suggestionfor its optimization. Therefore, optimization for proactive usage havenot been conducted.

The inventors of the present invention have found that high ratecharacteristics of the non-aqueous electrolyte battery can be improveddramatically by forming a resin layer possessing conductivity in thefollowing manner. First, a soluble nitrocellulose-based resincomposition is obtained by dispersing a conductive material in thesoluble nitrocellulose. Then, the resin layer having conductivity,including the soluble nitrocellulose-based resin and the conductivematerial is formed on the conductive substrate. The nitrogenconcentration of the soluble nitrocellulose of the present invention ispreferably 10 to 13%, and particularly preferably 10.5 to 12.5%. Whenthe nitrogen concentration is too low, there are cases where theconductive material cannot be dispersed sufficiently, depending on thetype of the conductive material. When the nitrogen concentration is toohigh, the soluble nitrocellulose becomes chemically unstable, and thusit would be dangerous to use it for the battery. Since the nitrogenconcentration depends on the number of the nitro group, adjustment ofthe nitrogen concentration is conducted by adjusting the number of nitrogroup. In addition, it is preferable that the viscosity of theafore-mentioned soluble nitrocellulose, which is measured in accordancewith JIS K-6703, is usually 1 to 6.5 seconds, particularly 1.0 to 6seconds; and the acid content is 0.006% or lower, particularly 0.005% orlower. When these values exceed these range, there are cases where thedispersibility of the conductive material and the batterycharacteristics lower.

The soluble nitrocellulose-based resin of the present invention cancontain the soluble nitrocellulose by 100 mass % (when the entire resincomponent is taken as 100 mass %), or other resin component may be usedin combination. When the other resin component is used in combination,it is preferable that the soluble nitrocellulose is contained by 40 mass% or more, more preferably 50 mass % or more, 90 mass % or less, andparticularly 80 mass % or less, with respect to the total resincomponent. Particular ratio of the soluble nitrocellulose is, forexample, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90 mass %, and may bein the range of two values selected from the values exemplified above.Through an investigation conducted for the internal resistance of theconductive resin layer prepared by adding a conductive material tovarious resins, it became apparent that when the soluble nitrocelluloseis contained by 50 mass % or more, the resistance of the resin layer canbe greatly reduced, sufficient high rate characteristics can beobtained, adhesion becomes superior, and the lifetime of the product canbe elongated. On the other hand, when the amount of solublenitrocellulose formulated is too small, improvement in dispersibility ofthe conductive material, which is obtained as an effect of formulatingthe soluble nitrocellulose, may not be obtained. It is assumed thataddition of 40 mass % or more of the soluble nitrocellulose cansufficiently lower the resistance of the resin layer.

The soluble nitrocellulose-based resin according to the presentinvention may be prepared by adding various resin to the afore-mentionedsoluble nitrocellulose. In the present invention, battery performance(including capacitor performance, hereinafter the same) was investigatedto find that it is preferable to add a melamine-based resin, anacryl-based resin, a polyacetal-based resin, or an epoxy-based resin incombination. By such addition, the battery performance can be improvedto a level equal to or higher than the case where the solublenitrocellulose is used as a resin component by 100 mass %. Each of theresin components will be described hereinafter. In the followingexplanation, the number average molecular weight and the weight averagemolecular weight are obtained by GPC (gel permeation chromatography).

It is assumed that the hardenability of the resin is improved, adhesionwith the conductive substrate is improved, and the battery performanceis improved, since the afore-mentioned melamine-based resin undergoes ahardening reaction with the soluble nitrocellulose. The amount of themelamine-based resin being added shall be, 5 to 200 mass %, morepreferably 10 to 150 mass %, when the soluble nitrocellulose as theresin component is taken as 100 mass %. When the amount added is lessthan 5 mass %, the effect is low. When the amount added exceeds 200 mass%, hardening reaction overly proceeds and the resin layer becomes toohard. This would cause detachment during the manufacture of batteries,and there may be a case where the discharge rate characteristicsdecrease. As the melamine-based resin, butylated melamine, isobutylatedmelamine, methylated melamine and the like can be preferably used forexample. The number average molecular weight of the melamine-based resinis, for example, 500 to 50,000, particularly for example 500, 1,000,2,000, 2,500, 3,000, 4,000, 5,000, 10,000, 20,000, or 50,000. The numberaverage molecular weight may be in the range of two values selected fromthe values exemplified above.

The afore-mentioned acryl-based resin has superior adhesion with aconductive substrate, especially with aluminum and copper. Therefore,addition of the acryl-based resin can improve the adhesion of thesoluble nitrocellulose-based resin with the conductive substrate. Theamount of the acryl-based resin being added shall be, 5 to 200 mass %,more preferably 10 to 150 mass %, when the soluble nitrocellulose as theresin component is taken as 100 mass %. When the amount added is lessthan 5 mass %, the effect is low. When the amount added exceeds 200 mass%, adverse effect is caused on the dispersibility of the conductivematerial. This may lead to a case where the discharge ratecharacteristics decrease. As the acryl-based resin, a resin containingacrylic acid, methacrylic acid, and derivatives thereof as a maincomponent, or an acrylic copolymer including such monomers canpreferably be used. In particular, methyl acrylate, ethyl acrylate,methyl methacrylate, isopropyl methacrylate and their copolymer can beused. In addition, acryl-based compounds having a polar group, such asacrylonitrile, methacrylonitrile, acryl amide, methacryl amide and thelike, and a copolymer thereof can preferably be used. The weight averagemolecular weight of the acryl-based resin is, for example, 30,000 to1,000,000, particularly for example 30,000, 40,000, 50,000, 60,000,70,000, 80,000, 90,000, 100,000, 150,000, 200,000, 300,000, 400,000,500,000, 600,000, 700,000, 800,000, 900,000, or 1,000,000. The weightaverage molecular weight may be in the range of two values selected fromthe values exemplified above.

The afore-mentioned polyacetal-based resin is superior in compatibilitywith the soluble nitrocellulose. Therefore, suitable flexibility can beprovided to the resin layer, and thus adhesion with the active materiallayer can be improved. The amount of the polyacetal-based resin beingadded shall be, 5 to 200 mass %, more preferably 20 to 150 mass %, whenthe soluble nitrocellulose as the resin component is taken as 100 mass%. When the amount added is less than 5 mass %, the effect is low. Whenthe amount added exceeds 200 mass %, adverse effect is caused on thedispersibility of the conductive material. This may lead to a case wherethe discharge rate characteristics decrease. As the polyacetal-basedresin, polyvinylbutyral, polyacetoacetal, polyvinylacetoacetal and thelike can preferably be used. The weight average molecular weight of thepolyacetal-based resin is, for example, 10,000 to 500,000, particularlyfor example 10,000, 20,000, 30,000, 40,000, 50,000, 60,000, 70,000,80,000, 90,000, 100,000, 150,000, 200,000, or 500,000. The weightaverage molecular weight may be in the range of two values selected fromthe values exemplified above.

Since the afore-mentioned epoxy-based resin is superior in adhesion withthe conductive substrate, the adhesion with the conductive substrate canbe further improved by adding the epoxy-based resin. The amount of theepoxy-based resin being added shall be, 5 to 200 mass %, more preferably10 to 150 mass %, when the soluble nitrocellulose as the resin componentis taken as 100 mass %. When the amount added is less than 5 mass %, theeffect is low. When the amount added exceeds 200 mass %, adverse effectis caused on the dispersibility of the conductive material. This maylead to a case where the discharge rate characteristics decrease. As theepoxy-based resin, glycidyl ether type resins such as bisphenol A typeepoxy resin, bisphenol F type epoxy resin, tetramethylbiphenyl type andthe like are preferable. The weight average molecular weight of theepoxy-based resin is, for example, 300 to 50,000, particularly forexample 300, 500, 1,000, 2,000, 3,000, 4,000, 5,000, 10,000, 20,000, or50,000. The weight average molecular weight may be in the range of twovalues selected from the values exemplified above.

In the present invention, the soluble nitrocellulose based resin maycontain the soluble nitrocellulose by 100% as the resin component, asdescribed above. Here, it is more preferable that at least one type ofthe afore-mentioned acryl-based resin and the polyacetal-based resin,the melamine-based resin, and the soluble nitrocellulose are contained.By such combination, the discharge rate characteristics and the longlife characteristics of the battery becomes particularly superior.

In addition, particularly when the total amount of the acryl-basedresin, polyacetal-based resin, melamine-based resin, and the solublenitrocellulose is taken as 100 mass %, it is further preferable that theamount of melamine-based resin is 5 to 55 mass %, and the amount ofsoluble nitrocellulose is 40 to 90 mass %. The amount of the acryl-basedresin or the polyacetal-based resin to be formulated is the resultingamount when the amount of the melamine-based resin and the solublenitrocellulose formulated is deducted from 100 mass %. In such case, thedischarge rate characteristics and the long life characteristics of thebattery becomes further superior. The amount of the melamine-based resinto be formulated is, in particular, 5, 10, 15, 20, 25, 30, 35, 40, 45,50, or 55 mass %. The amount may be in the range of two values selectedfrom the values exemplified above. The amount of the solublenitrocellulose to be formulated is, in particular, 40, 45, 50, 55, 60,65, 70, 75, 80, 85, or 90 mass %. The amount may be in the range of twovalues selected from the values exemplified above.

<2-2. Conductive Material>

The conductive resin layer of the present invention is provided inbetween the conductive substrate and the active material layer or theelectrode material layer. The conductive resin layer functions as apathway of electrons which moves between the conductive substrate andthe active material layer or the electrode material layer, and thuselectron conductivity is required. Since the solublenitrocellulose-based resin itself is high in insulation properties,conductive material must be formulated in order to impart the electronconductivity. As the conductive material used in the present invention,publicly known carbon powder, metal powder and the like can be used.Here, among them, carbon powder is preferable. As the carbon powder,acetylene black, Ketjen black, furnace black, carbon nanotube and thelike can be used. The amount of the conductive material to be added ispreferably 40 to 100 mass %, more preferably 40 to 80 mass % withrespect to 100 mass % of the resin component of the solublenitrocellulose-based resin (solid content, hereinafter the same). Whenthe added amount is less than 40 mass %, the volume resistivity of theresin layer becomes high, and when the added amount exceeds 100 mass %,the adhesion with the conductive substrate lowers. Conventional methodscan be used to disperse the conductive material into the resin componentsolution of the soluble nitrocellulose-based resin. For example, theconductive material can be dispersed by using a planetary mixer, a ballmill, a homogenizer, and the like.

<2-3. Water Contact Angle>

The water contact angle of the surface of the conductive resin layeraccording to the present invention is preferably 80 degrees or more and125 degrees or less, more preferably 90 degrees or more and 110 degreesor less. When the conductive material is merely added to the solublenitrocellulose-based resin to form the resin layer, there are caseswhere sufficient adhesion at the interface of the conductive substrateand the resin layer, interface of the resin layer and the activematerial layer, or the interface of the resin layer and the electrodematerial layer cannot be obtained. This is since the state of the resinlayer changes depending on the type of the resin and the conditions forforming the resin layer. Here, contact angle, which shows thewettability of a liquid, is a surface texture that possesses a largeinfluence to the adhesion. Therefore, by obtaining the contact angle ofwater, which has relatively large surface tension, adhesion of thecurrent collector and active material layer of the electrode materiallayer formed thereon can be evaluated. In this case, regarding the resinlayer and the water contact angle, it may seem that the smaller thewater contact angle is, the more the adhesion improves, and the more thedischarge rate can be improved. However, when the contact angle is toosmall, there is a possibility that adverse effect is caused on theadhesion of the conductive substrate and on the discharge ratecharacteristics. Therefore, it is necessary to regulate the watercontact angle in the present invention. This issue will also bediscussed later.

In the present specification, water contact angle is a value obtained byθ/2 method in a thermostatic chamber at 23° C. The water contact anglecan be obtained by using a contact angle meter. After forming a resinlayer on the current collector, a few micro liters of pure water isadhered as a droplet onto its surface, and then the water contact angleis observed. Since the surface tension of the water varies by the changein temperature, the water contact angle is observed in a thermostaticchamber at 23° C.

As a result of measuring the water contact angle by forming resin layersin accordance with various conditions, it became apparent that when thewater contact angle is 110 degrees or less, adhesion with the activematerial layer or the electrode material layer becomes particularlysuperior. In addition, an investigation on the relation of the watercontact angle and the adhesion between the conductive substrate and theresin layer was made by forming resin layers having a different watercontact angle. Accordingly, it became apparent that when the watercontact angle of the surface of the resin layer is 90 degrees or more,the discharge rate characteristics becomes particularly superior. Thereasons for such results are not clear, however, it is assumed that suchdifference is due to the subtle difference in the adhesion state of theconductive substrate and the resin layer. Therefore, it is especiallypreferable that the water contact angle is 90 degrees or more. Asdescribed, the regulation of the water contact angle according to thepresent invention has been made in view of not only the adhesion of thesoluble nitrocellulose-based resin with the active material layer or theelectrode material layer, but also in view of the adhesion of theconductive substrate with the resin layer. The current collector of thepresent invention thus regulated with its water contact angle canprovide suitable discharge rate characteristics when used as anelectrode structure for batteries and electrical storage device.

Regarding the current collector of the present invention, there is noparticular limitation in the method for forming the conductive resinlayer. Here, it is preferable to form the conductive layer by applying asoluble nitrocellulose-based resin material (solution, dispersion, orpaste) including a soluble nitrocellulose-based resin and a conductivematerial onto a conductive substrate, followed by baking. As the methodfor coating, a roll coater, a gravure coater, a slit dye coater and thelike can be used. In a case where the resin layer is formed by coating,the baking temperature, as the final temperature of the conductivesubstrate, is preferably 100 to 250° C., and the baking time ispreferably 10 to 60 seconds. When the baking temperature is lower than100° C., the soluble nitrocellulose-based resin would not hardensufficiently, and when the baking temperature exceeds 250° C., there arecases where the adhesion with the active material layer decreases. Inaddition, when the baking time is shorter than 10 seconds, the solventwould vaporize before the resin hardens, thereby causing defects in theresin layer. When the baking time exceeds 60 seconds, some temperatureconditions may cause the foil to soften and thus results in insufficientstrength.

In general, the water contact angle tends to become large as the bakingtemperature is raised and the baking time is made longer. Therefore, inorder to obtain a resin layer having the water contact angle within theafore-mentioned range, the resin layer is formed with a particularcondition first, and then the water contact angle of the resin layerthus formed is measured. When the water contact angle measured issmaller than the afore-mentioned lower limit, the baking temperature israised or the baking time is made longer. When the water contact anglemeasured is larger than the afore-mentioned upper limit, the bakingtemperature is reduced or the baking time is made shorter. Accordingly,the conditions are adjusted. The value of the water contact angle cannotbe determined merely by the composition of the solublenitrocellulose-based resin and the baking temperature, however, thewater contact angle can be adjusted to the desired value by conductingseveral trial and errors, when the afore-mentioned method is used.

By using the current collector of the present invention, sufficientadhesion in the interface of the resin layer and the active materiallayer or in the interface of the resin layer and the electrode materiallayer can be obtained even when the active material layer of theelectrode material layer is formed and is immersed in an electrolytesolution. In addition, sufficient adhesion can be obtained in theinterface of the resin layer and the conductive substrate. Further,large detachment is not observed even after charge and discharge isrepeated. Accordingly, a current collector having sufficient adhesionand superior discharge rate characteristics can be obtained.

The thickness of the resin layer can be adjusted in accordance with theapplication of the electrode. Here, the thickness is preferably 0.1 to 5μm, particularly preferably 0.3 to 3 μm. When the thickness is less than0.1 μm, portions where the conductive substrate is not covered wouldappear, resulting in insufficient battery characteristics. When thethickness exceeds 5 μm, the active material layer must be made thin forsuch excess in the thickness when structuring a battery. This wouldresult is insufficient capacity density and would make it difficult tomanage with the downsizing of batteries, capacitors and the like. Inaddition, concerning the application in square battery, when theelectrode structure is wound together with a separator, cracks areformed in the resin layer at the most inner portions of winding havingan extremely small radius of curvature. This would lead to generation ofdetachment from the active material layer.

In order to improve the adhesion of the surface of the conductivesubstrate, it is effective to perform a pretreatment to the conductivesubstrate onto which the conductive resin layer is formed. When a metalfoil manufactured by rolling is used as the substrate, there are caseswhere rolling oil and wear debris are left on the surface. In suchcases, adhesion can be improved by removing them. In addition, adhesioncan be improved by performing a dry activation treatment such as coronadischarge treatment.

<3. Electrode Structure>

The electrode structure of the present invention can be obtained byforming an active material layer or an electrode material layer on oneside or both sides of the conductive substrate. An electrode structurefor an electrical storage device provided with the electrode materiallayer will be described later.

The active material layer may be a obtained by formulating an activematerial in a soluble nitrocellulose-based resin, thereby allowing theconductive resin layer to act as an active material layer, or may beobtained by forming another layer on the conductive resin layer. Inaddition, a non-aqueous electrolyte solution battery can be manufacturedwith the electrode structure, a separator, a non-aqueous electrolytesolution and the like. In the electrode structure for the non-aqueousbattery and the non-aqueous electrolyte solution battery of the presentinvention, conventional parts for non-aqueous battery can be used as theparts other than the current collector.

The active material layer formed in the present invention may be theones conventionally proposed for the non-aqueous electrolyte battery.For example, positive electrode structure of the present invention canbe obtained by coating the current collector of the present inventionwhich uses aluminum foil with a paste. Here, the coating method is aconventional method, and the paste for the positive electrode structureis obtained by using LiCoO₂, LiMnO₂, LiNiO₂ and the like as an activematerial and using carbon black such as acetylene black and the like asa conductive material, and dispersing the active material and theconductive material in PVDF, CMC (carboxymethyl cellulose, hereinafterthe same) and the like as a binder. Such paste can be coated on a copperfoil or an aluminum foil, and is suitable for the positive electrodestructure.

The negative electrode structure of the present invention can beobtained by coating a paste onto the current collector of the presentinvention using a copper foil. Here, black lead (graphite), graphite,mesocarbon microbead and the like is used as the active material. Thepaste can be obtained by dispersing the active material in CMC as athickening agent, and then mixing the resulting dispersion with SBR as abinder. Such paste can be coated on a copper foil or an aluminum foil,and is suitable for the negative electrode structure.

<4. Usage of Electrode Structure>

The electrode structure which uses the current collector of the presentinvention can be used in various applications as electrodes. Forexample, it can be used as a non-aqueous electrolyte battery, anelectrical double layer capacitor, a lithium ion capacitor or anelectrical storage device.

<4-1. Non-Aqueous Electrolyte Battery>

A separator is sandwiched in between the positive electrode structureand the negative electrode structure to constitute the non-aqueouselectrolyte battery of the present invention. Here, the separator isimmersed in an electrolyte for a non-aqueous electrolyte battery,containing a non-aqueous electrolyte. As the non-aqueous electrolyte andthe separator, conventional ones used for the non-aqueous electrolytebattery can be used. For example, as the solvent of the electrolyte,carbonates, lactones and the like can be used. Here, LiPF₆ or LiBF₄ aselectrolytes dissolved in a mixture of EC (ethylene carbonate) and EMC(ethylmethyl carbonate) can be used. As the separator, a membrane madeof polyolefin having microporous can be used for example.

<4-2. Electrical Storage Device (Electrical Double Layer Capacitor,Lithium Ion Capacitor and the Like)

The electrode of the present invention can be used for an electricaldouble layer capacitor and a lithium ion capacitor, which have a demandfor longer lifetime with the ability to charge and discharge at a largecurrent density. Here, the electrode of the present invention can bemanufactured by a conventional method. In the electrical double layercapacitor, electrode material layer of both of the positive electrodeand the negative electrode are usually structured with an electrodematerial, a conductive material and a binder. In contrast, theelectrical double layer capacitor of the present invention can bestructures with the afore-mentioned electrode structure, a separator, anelectrolyte solution and the like. In the electrode structure for theelectrical double layer capacitor and the electrical double layer of thepresent invention, conventional parts for the electrical double layercapacitor can be used regarding the parts other than the electrodes. Inparticular, an electrode material used conventionally for the electricaldouble layer capacitor can be used as the electrode material. Forexample, carbon powder such as activated charcoal and black lead(graphite), or carbon fiber can be used. As the conductive material,carbon black such as acetylene black can be used. As the binder, PVDFand SBR can be used for example. The electrical double layer capacitorcan be structured by sandwiching a separator in between the electrodestructures of the present invention, and then immersing the separator inthe electrolyte solution. As the separator, a membrane made ofpolyolefin having microporous, a non-woven fabric for an electricaldouble layer capacitor and the like can be used for example. Regardingthe electrolyte solution, carbonates and lactones can be used as thesolvent for example, and tetraethylammonium salt, triethylmethylammoniumsalt and the like can be used as the electrolyte, andhexafluorophosphate, tetrafluoroborate and the like can be used as thenegative ion. Lithium ion capacitor is structured by combining anegative electrode of a lithium ion battery and a positive electrode ofan electrode double layer capacitor.

EXAMPLES

<1. Preparation of Current Collector>

A resin solution was prepared by dissolving a resin shown in Table 1 inan organic solvent of MEK, by the ratio shown in Table 1. Acetyleneblack was added to the resin solution by 60 mass % with respect to theresin component (solid content of the resin, hereinafter the same), andthen the mixture was dispersed using a ball mill for 8 hours to obtain acoating. The coating thus obtained was coated on one side of an aluminumfoil with a thickness of 20 μm (JIS A1085) by a bar coater. The coatingwas heated for 30 seconds so that the final substrate temperaturereaches the baking temperature shown in Table 1, to prepare the currentcollector. The thickness of the resin layer after baking was as shown inTable 1.

In Table 1, the weight of the soluble nitrocellulose is the weight ofthe solid content. In addition, the details of the resin provided in anabbreviated manner in Table 1 is shown in Table 2.

TABLE 1 Resin 1 2 3 Form- Form- Form- ulation ulation ulation WeightAmount Weight Amount Number Amount Application Conditions Average (partsAverage (parts Average (parts Baking Molecular by Molecular by Molecularby Temperature Thickness Type Weight mass) Type Weight mass) Type Weightmass) (° C.) (μm) Example 1 Soluble — 80 — — — Melamine 1 2700 20 1800.05 Nitro- cellulose 1 2 Soluble — 80 — — — Melamine 1 2700 20 180 1.1Nitro- cellulose 1 3 Soluble — 80 — — — Melamine 1 2700 20 180 4.8Nitro- cellulose 1 4 Soluble — 80 — — — Melamine 1 2700 20 180 5.5Nitro- cellulose 1 5 Soluble — 100 — — — — — — 180 2.2 Nitro- cellulose2 6 Soluble — 80 — — — Melamine 1 2700 20 180 2.3 Nitro- cellulose 3 7Soluble — 40 Epoxy 1 2900 40 Melamine 2 2100 20 180 2.1 Nitro- cellulose1 8 Soluble — 54 Polyacetal 1 90000 16 Melamine 2 2100 30 180 2.3 Nitro-cellulose 1 9 Soluble — 54 Polyacetal 2 27000 16 Melamine 2 2100 30 1802.4 Nitro- cellulose 1 10 Soluble — 64 Polyacetal 2 27000 21 Melamine 22100 15 180 2.1 Nitro- cellulose 1 11 Soluble — 54 Acryl 1 70000 16Melamine 2 2100 30 180 2.1 Nitro- cellulose 1 12 Soluble — 64 Acryl 170000 21 Melamine 3 2500 15 180 2.2 Nitro- cellulose 1 13 Soluble — 64Acryl 1 70000 21 Melamine 3 2500 15 90 2.2 Nitro- cellulose 1 14 Soluble— 64 Acryl 1 70000 21 Melamine 3 2500 15 130 2.2 Nitro- cellulose 1 15Soluble — 64 Acryl 1 70000 21 Melamine 3 2500 15 180 2.2 Nitro-cellulose 1 16 Soluble — 64 Acryl 1 70000 21 Melamine 3 2500 15 250 2.2Nitro- cellulose 1 17 Soluble — 75 Acryl 2 80000 19 Melamine 1 2700  6180 2.3 Nitro- cellulose 1 18 Soluble — 60 Polyacetal 1 90000 40 — — —180 2.4 Nitro- cellulose 2 19 Soluble — 60 Acryl 2 80000 40 — — — 1802.4 Nitro- cellulose 2 20 Soluble — 60 Epoxy 2 900 40 — — — 180 2.3Nitro- cellulose 2 21 Soluble — 60 Pyromellitic — 40 — — — 180 2.1Nitro- Acid cellulose 2 22 Soluble — 60 Isophthalic — 40 — — — 180 2.3Nitro- Acid cellulose 2 23 Soluble — 60 Acrylonitrile — 40 — — — 180 2.2Nitro- cellulose 2 Comparative 1 Ethyl 70000 100 — — — — — — 180 2.3Example Cellulose 2 Ethyl 70000 70 Polyacetal 1 90000 30 — — — 180 2.3Cellulose 3 Ethyl 70000 70 Acryl 1 70000 30 — — — 180 2.1 Cellulose 4Ethyl 70000 70 Epoxy 1 2900 30 — — — 180 2.2 Cellulose 5 Methyl 70000 70— — — Melamine 1 2700 30 180 2.1 Cellulose 6 Methyl 70000 50 Polyacetal1 90000 30 Melamine 1 2700 20 180 2.3 Cellulose 7 Methyl 70000 50 Acryl1 30 Melamine 1 2700 20 180 2.4 Cellulose 8 Methyl 70000 50 Epoxy 1 290030 Melamine 1 2700 20 180 2.5 Cellulose Adhesion Resistance BetweenDischarge Rate Characteristics Lifetime of Resin Resin Grade ofElectrical Double Layer Lithium Layer Layer and Contact Discharge RateLithium ion Battery Capacitor ion Electrical Double (mΩ) Aluminum AngleCharacteristics 5 C 10 C 20 C 100 C 300 C 500 C Battery Layer CapacitorExample 1 64 E 93 E 83 71 68 92 75 68 C C 2 78 D 96 D 86 78 71 93 76 73B B 3 189 D 96 D 86 77 72 82 77 73 B B 4 253 E 95 E 84 72 66 81 74 67 CC 5 56 E 98 E 83 72 67 90 73 66 C C 6 92 D 102 D 87 79 73 93 78 74 B B 7423 C 108 C 88 83 76 95 81 78 B B 8 261 B 193 B 93 86 80 97 86 83 A A 9263 A 98 B 94 87 83 97 87 85 A A 10 240 A 97 A 97 93 87 99 93 89 A A 11176 A 104 B 91 88 82 96 86 84 A A 12 154 A 101 A 98 94 89 98 94 88 A A13 155 C 85 B 92 85 80 96 87 81 A A 14 167 B 90 A 96 92 86 99 93 87 A A15 156 A 110 A 97 93 88 99 94 88 A A 16 153 A 117 B 91 87 81 97 58 84 AA 17 164 A 95 A 98 95 88 98 94 87 A A 18 142 C 99 C 87 82 77 95 83 76 BB 19 128 C 97 C 89 81 78 94 82 78 B B 20 199 E 109 E 84 71 66 90 74 68 CC 21 128 D 104 D 86 77 72 93 77 71 C C 22 131 D 102 D 85 78 73 92 76 72C C 23 118 D 103 D 86 78 70 93 78 73 C C Comparative 1 2530 B 98 G 72 6151 88 63 54 D D Example 2 3180 B 105 G 73 62 50 87 62 53 D D 3 3210 B 97G 73 61 52 86 63 57 D D 4 4040 B 108 G 72 63 53 87 61 54 D D 5 2660 A106 F 76 69 56 86 67 56 D D 6 3340 A 103 F 78 68 56 88 68 59 D D 7 3290A 101 F 77 67 57 88 66 58 D D 8 3410 A 107 F 76 67 55 56 69 57 D D

TABLE 2 Abbreviated Name in Table 1 Details Soluble JIS K6703L1/4Nitrocellulose 1 Soluble JIS K6703L1/8 Nitrocellulose 2 Soluble JISK6703H1/4 Nitrocellulose 3 Epoxy 1 Bisphenol A type Epoxy Epoxy 2Bisphenol F type Epoxy Acryl 1 Acryl Copolymer (MethylAcrylate:Methacrylic Acid = 95:5) Acryl 2 Acryl Copolymer (AcrylAmide:Hydroxyalkyl Acrylate:Acrylic Acid = 40:50:10) Melamine 1Butylated Melamine Melamine 2 Methylated Melamine Melamine 3Isobutylated Melamine Polyacetal 1 Polyvinyl Butyral Polyacetal 2Polyvinyl Acetoacetal<2. Evaluation><2-1. Measurement of the Resistance of Resin Layer of Current Collector,Measurement of Water Contact Angle, and Evaluation of Adhesion BetweenSubstrate and Resin Layer>

The thickness, resistance, and water contact angle or the resin layer ofthe current collector, and the adhesion between the substrate and theresin layer was evaluated. The results are shown in Table 1.

Regarding the thickness of the resin layer, film thickness measuringmachine “HAKATTARO G” (available from SEIKO-em) was used to calculatethe thickness of the resin layer as a difference in the thicknessbetween the portion formed with the resin layer and the portion withoutthe resin (portion only with the aluminum foil).

The resistance of the resin layer was measured as follows. A sample wasplaced on a surface plate with the surface having the coating facingupward, and then a 20 mm-cube block made of copper was placed on thecoating (the surface which comes in contact with the coating was mirrorfinished). The electrical resistance between the copper block and thealuminum foil was measured with the condition in which a load of 700 gfwas applied.

Water contact angle was obtained using a contact angle meter (DropMaster DM-500, available from Kyowa Interface Science Co., LTD.). First,1 μl of water droplets were adhered on the surface of the resin layer ina thermostatic chamber at 23° C., and then the contact angle after 2seconds was measured by θ/2 method. Adhesion was evaluated by theconditions of detachment when Cellotape (available from NICHIBAN CO.,LTD.) was attached on the surface of the resin layer, and was thenpeeled off at once.

-   A: No Detachment-   B: Approximately ¼ Detached-   C: Approximately ½ Detached-   D: Approximately ¾ Detached-   E: Whole Surface Detached    <2-2. Evaluation of Discharge Rate Characteristics and Electrode    Lifetime of Lithium Ion Battery>

Lithium ion batteries were prepared by using the current collectorprepared by the afore-mentioned method. The discharge ratecharacteristics and the battery lifetime were evaluated in accordancewith the following manner. The results are shown in Table 1.

(Preparation of Lithium Ion Battery)

A positive electrode was prepared as follows. A paste was prepared bydispersing LiCoO₂ as an active material and acetylene black as aconductive material in PVDF (polyvinylidene fluoride) as a binder. Thepaste thus obtained was coated on the current collector electrode sothat the thickness of the coating is 70 μm, to give the positiveelectrode. A negative electrode was prepared as follows. A paste wasprepared by dispersing black lead (graphite) as an active material inCMC (carboxymethyl cellulose), followed by the addition of SBR (styrenebutadiene rubber) as a binder. The paste thus obtained was coated on acopper foil with a thickness of 20 μm so that the thickness of thecoating is 70 μm, to give the negative electrode. A microporousseparator made of polypropylene was sandwiched by these electrodestructures, and was then cased in the battery casing to obtain a coinbattery. A 1 mol/L solution of LiPF₆ in a solvent mixture of EC(ethylene carbonate) and EMC (ethylmethyl carbonate) was used as theelectrolyte solution.

(Method for Evaluating Discharge Rate Characteristics)

Discharge capacity of these lithium ion batteries (based on 0.2C, unit%) was observed for the discharge current rate of 1C, 5C, 10C, and 20C,when the upper voltage limit of charged state was 4.2 V, charge currentwas 0.2C, discharge final voltage was 2.8 V, and the temperature was 25°C. (Here, 1C is the value of the current A) when the current capacity(Ah) of the battery is taken out in 1 hour (h). At 20C, the currentcapacity of the battery can be taken out in 1/20h=3 min. On the otherhand, the battery can be charged in 3 minutes.)

(Method for Evaluating Lifetime of Electrode)

The battery was first charged at an electrolyte solution temperature of40° C., upper limit voltage of 4.2V, and a charging current of 20C. Thenthe battery was discharged to a final voltage of 2.8V, at a dischargingcurrent of 20C. Number of cycles when the discharge capacity reaches 60%of the discharge capacity of the first cycle was observed (maximum 500cycles), and was evaluated in accordance with the following criteria.

-   A: 500 cycles or more-   B: 450 cycles or more and less than 500 cycles-   C: 400 cycles or more and less than 450 cycles-   D: less than 400 cycles    <2-3. Evaluation of Discharge Rate Characteristics and Electrode    Lifetime of Electrical Double Layer Capacitor>

Electrical double layer capacitors were prepared by using the currentcollector prepared by the afore-mentioned method. The discharge ratecharacteristics and the battery lifetime were evaluated in accordancewith the following manner. The results are shown in Table 1.

(Preparation of Electrical Double Layer Capacitor)

A paste was prepared by dispersing activated charcoal as an electrodematerial and Ketjen black as a conductive material in PVDF as a binder.The paste thus obtained was coated on the current collector electrode sothat the thickness of the coating is 70 μm, to give the positive andnegative electrode structures. A non-woven fabric for an electricaldouble layer capacitor immersed in the electrolyte solution wassandwiched and fixed by two of these electrode structures, and thus theelectrical double layer capacitor was structured. A solution obtained byadding 1.5 mol/L solution of TEMA (triethylmethyl ammonium) andtetrafluoroboric acid in propylene carbonate as a solvent was used asthe electrolyte solution.

(Method for Evaluating Discharge Rate Characteristics)

Discharge capacity of these electrical double layer capacitors (based on1C, unit %) was observed for the discharge current rate of 100C, 300C,and 500C, when the upper voltage limit of charged state was 2.8 V,charge current was 1C, condition for the completion of charging was 2hours, discharge final voltage was 0 V, and the temperature was 25° C.

(Method for Evaluating Lifetime of Electrode)

The capacitor was first charged at an electrolyte solution temperatureof 40° C., upper limit voltage of 2.8V, and a charging current of 500C.Then the battery was discharged to a final voltage of 0V, at adischarging current of 500C. Number of cycles when the dischargecapacity reaches 80% of the discharge capacity of the first cycle wasobserved (maximum 5000 cycles), and was evaluated in accordance with thefollowing criteria.

-   A: 5000 cycles or more-   B: 4500 cycles or more and less than 5000 cycles-   C: 4000 cycles or more and less than 4500 cycles-   D: less than 4000 cycles    <2-4. Conclusion>

From the results shown in Table 1, it can be concluded that Examples 1to 23, provided with a conductive resin layer, have superior dischargerate characteristics and battery lifetime, when compared withComparative Examples 1 to 8, provided with a resin layer of ethylcellulose.

In addition, when the results of Examples 8 to 17 are compared with theresults of other Examples, it can be concluded that when the solublenitrocellulose-based resin contains at least one of the resins selectedfrom the group consisting of a melamine-based resin, an acryl-basedresin, a polyacetal-based resin, and an epoxy-based resin; in additionto a soluble nitrocellulose, highly superior results can be obtained inboth of the discharge rate characteristics and the battery lifetime.

The invention claimed is:
 1. A positive electrode structure of lithiumion battery, comprising: a conductive substrate, a conductive resinlayer provided on one side or both sides of the conductive substrate;and an active material comprising lithium metal oxide; wherein theconductive resin layer contains a soluble nitrocellulose-based resin anda conductive material, wherein the soluble nitrocellulose-based resincomprises a soluble nitrocellulose, wherein a nitrogen concentration ofthe soluble nitrocellulose is 10 to 13%, and at least one resin selectedfrom the group consisting of a melamine-based resin, an acryl-basedresin, a polyacetal-based resin, and an epoxy-based resin, and thesoluble nitrocellulose is contained by 40 to 90 mass % when the total ofthe at least one resin and the soluble nitrocellulose is 100 mass %. 2.The positive electrode structure of claim 1, wherein the solublenitrocellulose-based resin comprises the melamine-based resin, thesoluble nitrocellulose, and at least one resin selected from the groupconsisting of the acryl-based resin and the polyacetal-based resin. 3.The positive electrode structure of claim 2, wherein the melamine-basedresin is-contained by 5 to 55 mass %.
 4. The positive electrodestructure of claim 1, wherein a surface of the conductive resin layerhas a water contact angle of 80 degrees to 125 degrees when measured byθ/2 method in a thermostatic chamber at 23° C.
 5. The positive electrodestructure of claim 4, wherein the water contact angle is 90 degrees to110 degrees.
 6. The positive electrode structure of claim 1, furthercomprising: an active material layer provided on the conductive resinlayer.
 7. The positive electrode structure of claim 1, wherein theconductive resin layer contains the active material.
 8. A lithium ionbattery, comprising the positive electrode structure of claim 1.